** For Jacobson spaces, closed points see everything about the topology. **

Lemma 5.18.7. Suppose $X$ is a Jacobson topological space. Let $X_0$ be the set of closed points of $X$. There is a bijective, inclusion preserving correspondence

\[ \{ \text{finite unions loc.\ closed subsets of } X\} \leftrightarrow \{ \text{finite unions loc.\ closed subsets of } X_0\} \]

given by $E \mapsto E \cap X_0$. This correspondence preserves the subsets of locally closed, of open and of closed subsets.

**Proof.**
We just prove that the correspondence $E \mapsto E \cap X_0$ is injective. Indeed if $E\neq E'$ then without loss of generality $E\setminus E'$ is nonempty, and it is a finite union of locally closed sets (details omitted). As $X$ is Jacobson, we see that $(E \setminus E') \cap X_0 = E \cap X_0 \setminus E' \cap X_0$ is not empty.
$\square$

## Comments (1)

Comment #1117 by Simon Pepin Lehalleur on

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